The recovery of hindlimb stepping in complete spinally transected (ST) animals is improved by treadmill training. Little is known about the mechanisms by which treadmill training improves stepping, but our studies suggest that training enhances plasticity in the neurons of the lumbar spinal cord that generate stepping. Brain derived neurotrophic factor (BDNF) is a member of the neurotrophin family and has been shown to play a crucial role in activity-dependent plasticity in the central nervous system. We propose to study the role of BDNF in activity-dependent plasticity in the spinal cord of ST rats that undergo hindlimb treadmill training. Our overall hypothesis is that endogenously-produced BDNF improves the ability of ST rats to generate hindlimb stepping by strengthening synapses within the neural circuits that control stepping. In the first experiment, we will first determine the extent that BDNF synthesis by lumbar spinal neurons is dependent on the amount of treadmill training imposed on the ST rats. We will use a robotic device that, unlike conventional treadmills, allows precise control over the amount of hindlimb activity during treadmill stepping. Using a combination of in situ hybridization and immunohistochemical techniques, we will then examine the expression of BDNF mRNA and protein, TrkB (the receptor for BDNF) and plasticity-related molecules, synapsin I, CREB, CaMKII and NMDA receptor subunits in lumbar spinal neurons following 1 day, 4 or 12 weeks of treadmill training. In our second experiment, we will selectively enhance the activity of extensor neurons in one hindlimb using the robotic device to apply a force during treadmill training. After 4 weeks, we will compare the expression of BDNF mRNA and protein to determine if the more active neurons have higher BDNF expression. In our third experiment, we will administer drugs that block BDNF activity in the spinal cord to determine if the effects of treadmill training require BDNF action. Thus, at the end of the project, we will know the extent that BDNF synthesis in lumbar spinal neurons is dependent on the levels of hindlimb activity and we will know if BDNF activity is crucial for the recovery of stepping in ST rats. These studies will provide some important insight into the mechanisms by which treadmill training improves stepping after spinal cord injury. In addition, the findings may suggest a role for activity-based strategies in neurotrophin treatments for spinal cord injury.